Recently, it has been pointed out that ultraviolet ray in the UV-A region (320 to 400 nm) penetrates deep into the skin, becoming the main causative factor for photoaging and skin cancer. In view of this, the demand has also risen for cosmetics with a UV-protective effect in the UV-A region.
Conventionally, ultraviolet absorbers such as 2-ethylhexyl paramethoxycinnamate and inorganic powders such as titanium dioxide and zinc oxide have been used for protection against ultraviolet ray. Among them, zinc oxide has recently been frequently used in cosmetics for its relatively high shielding ability against not only the UV-B region (290 nm to 320 nm) but also the UV-A region.
However, when zinc oxide was used in an oil-in-water emulsified cosmetic composition, a problem arises that it was difficult to be blended into the cosmetic composition owing to its poor dispersibility in an aqueous system. Also, even when zinc oxide was used in combination with water-soluble polymers to increase the dispersion stability, a problem arises that zinc ions dissolved from zinc oxide caused aggregation of the water-soluble polymers, dramatically reducing long-term stability and feeling upon application of the cosmetic composition.
As a solution to the aforementioned problems, for example, there has been proposed an oil-in-water emulsified cosmetic composition having favorable powder dispersibility and long-term stability and an excellent feeling upon application obtained by using a copolymer of acryloyldimethyl taurate and hydroxyethyl acrylate and a powder such as titanium dioxide (refer to Patent Document 1). However, a problem thereof is that unless a specific acrylic silicone is used in combination as a dispersant in the oil-in-water emulsified cosmetic composition, the powder dispersibility becomes poor, and the acrylic silicone adversely produces a tight skin feeling, impairing the feeling upon application.
Also, there has been proposed an oil-in-water emulsified cosmetic composition having favorable long-term stability and excellent feeling upon application, transparency, and water resistance obtained by using zinc oxide, a cross-linking copolymer of acrylamide and 2-acrylamido-2-methylpropanesulfonic acid and/or an acrylic acid-sodium acryloyldimethyl taurate copolymer, a surfactant having an HLB of 10 or more, silicone oil, and water (refer to Patent Document 2). However, it has insufficient transparent feeling when applied and also poor spreadability, and thus is not satisfactory in terms of usability.
Meanwhile, there has been proposed a skin external agent having improved transparency and UV-protective effect obtained by using a flaky powder of zinc oxide (refer to Patent Document 3).